1. Field of the Invention
The invention relates to circuit interrupters having means for electronically analyzing the electrical conditions on the circuit being protected, and means for automtically opening to interrupt the current flow whenever electrical conditions exceed predetermined limits.
2. Description of the Prior Art
Circuit breakers are widely used in industrial and commercial applications for protecting electrical conductors and apparatus connected thereto from damage due to excessive current flow. Circuit breakers were initially designed to interrupt when the current flowing through them exceeded a certain level. Gradually, however, more elaborate time-current interrupting characteristics were required such that a circuit breaker would rapidly open upon very high overload conditions but would delay interruption upon detection of lower overload currents, the delay time being roughly inversely proportional to the degree of overload. Additionally, circuit breakers were called upon to interrupt upon the detection of ground fault currents. As the complexity of electrical distribution systems increased, the control portions of circuit breakers on a system were interconnected to provide selectivity and coordination of interruption sequences. This allowed the system designer to specify the order in which the various circuit breakers would interrupt under specified fault conditions.
During the late 1960's, solid-state electronic control circuits were developed for use in high power, low voltage circuit breakers. These control circuits performed functions such as instantaneous and delayed tripping which were traditionally achieved by magnetic and thermal means. The improved accuracy and flexibility of the solid state electronic controls resulted in their wide-spread acceptance, even though the electronic control circuits were often more expensive than their mechanical counterparts.
The earliest electronic control circuit designs utilized discrete components such as transistors, resistors, and capacitors. More recent designs have included integrated circuits which have provided improved product performance at reduced cost.
As the cost of energy continues its rapid rise, there is increasing interest in more effectively controlling the usage of electrical energy through the design of more sophisticated electrical distribution systems. Therefore, there is required a circuit breaker providing a more complex analysis of electrical conditions on the circuit being protected and even greater capability for coordination with other breakers. As always, it is extremely desirable to provide this capability at the same or lower cost.